/in/foo.cc: In function 'int main()':
/in/foo.cc:8:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s.size();i++)
              ~^~~~~~~~~
/in/foo.cc:9:9: warning: array subscript has type 'char' [-Wchar-subscripts]
   a[s[i]]++;
         ^